1. Field of the Invention
The present invention relates to a high-output semiconductor laser for use in optical disk drives and optical communication devices, and a method of manufacturing such a high-output semiconductor laser.
2. Description of the Related Art
When the optical output of a semiconductor laser reaches a certain level, the exit end of the semiconductor laser suffers catastrophic optical damage (COD). Therefore, the optical output of the semiconductor laser is limited below a certain value (COD optical output). The COD occurs when the optical density in the semiconductor reaches an inherent value (COD optical density) of the semiconductor.
One way of increasing the COD optical output of a semiconductor laser is to coat the exit end thereof with a film such as of a dielectric material. When such a coating is applied to the exit end to lower the reflectivity of the exit end, the optical density in the semiconductor waveguide is lowered thereby to increase the COD optical output. As Hakki and Nash reported in Journal of Applied Physics, Vol. 45, No. 9, pages 3907-3912, 1974, the exit end reflectivity R.sub.1 and the COD optical output are related to each other according to the following equation (1): ##EQU1##
(the equation (4) in the above literature) where
P.sub.coated : the COD optical output of a laser having an exit end coating with a reflectivity R.sub.1 : PA1 P.sub.uncoated : the COD optical output of a laser having no exit end coating PA1 R.sub.1 : the exit end reflectivity; and PA1 n.sub.eff : the effective refractive index of the semiconductor waveguide.
After the report by Hakki and Nash, the above equation has been used so far in designing high-output semiconductor lasers. The left side of the above equation (1): ##EQU2##
will be referred to as a COD index.
According to the equation by Hakki and Nash, the only way to increase the COD index is reducing the reflectivity. Under limit conditions where the reflectivity is infinitely small, the COD index converges to a maximum value n.sub.eff. Therefore, it is impossible to increase the COD index beyond n.sub.eff. However, if the reflectivity R.sub.1 is reduced, an oscillation threshold carrier density increases, degrading the oscillation characteristics of the semiconductor laser, e.g., resulting in an increased oscillation threshold current and degraded temperature characteristics.
Japanese laid-open patent publication No. 8-307004 discloses the relationship between the refractive index of a coating film of a semiconductor laser and the optical density in the semiconductor. FIG. 6 of this publication shows that if the refractive index of the coating film is greater than a certain value, the reflectivity increases and the optical density in the semiconductor decreases. The thickness (d.sub.1.sup.A) of the coating film, the refractive index (n.sub.1.sup.A) thereof, and the laser beam wavelength (.lambda.) fulfill the following equation: ##EQU3##
For example, if the refractive index is 1.3, then the reflectivity is 0.10, and the optical density (.vertline.E.sub.A.vertline..sup.2) in the semiconductor is 0.6. If the refractive index is 2.5, then the reflectivity is 0.10, but the optical density (.vertline.E.sub.A.vertline..sup.2) in the semiconductor has a smaller value of 0.2. Thus, the results disclosed in the above publication are obviously different from the results produced by Hakki and Nash.
The above publication describes operation and embodiments of a coating film comprising at least two dielectric films. According to the publication, the problems to be solved are that LDs of the Fabry-Perot design where an end film comprises a single dielectric film is generally unable to satisfy both a condition to obtain the reflectivity of the LD exit end which is required from the standpoint of designing LDs and a condition to minimize the photoelectric intensity at the interface between a semiconductor material and the end film which is required to avoid COD. However, the publication is silent as to whether the minimum value of the photoelectric intensity by a coating film composed of two dielectric films according to the embodiment is smaller than the minimum value of the photoelectric intensity by a single dielectric film.
Japanese laid-open patent publication No. 8-307004 suggests that a coating film comprising a single film may exhibit a COD index greater than the equation by Hakki and Nash. However, any optimum conditions between the coating film thickness and the refractive index are not revealed, and no comparison with the report by Hakki and Nash is disclosed.
As described above, any optimum conditions for the coating of the dielectric film relative to the COD optical output of semiconductor lasers are not clear, and it is difficult optimize semiconductor lasers to be designed.